This invention relates generally to an optical power isolator that permits the effective collection of optical power emanating from a laser and at the same time effectively isolates the laser from the rest of a communications link. The device is suitable for both direct and coherent optical detection communications and is especially useful in reducing laser noise in high speed analog links.
The stability of the output intensity and spectral characteristics of laser-based optical systems is critically important at high bit rates. Present and future systems for communication and sensor systems are sensitive to variations in these properties. A primary cause of transmission instability is feedback into the laser cavity. This feedback may be due to internal reflection from system optics. In military applications, disruptive signals may be intentionally introduced by opposing countermeasures. Reflections from the optical system can contribute as much as 30 dB of noise to a system. Much higher intensity interference may result from deliberate jammer techniques.
Although optical isolators are commonly used in transmission paths linking lasers to optical fiber channels, present systems are not totally satisfactory. Most conventional isolators use technology based on polarizing optical elements. Commercial devices provide approximately 25 dB of isolation, and they exhibit insertion losses of up to 5 dB. These devices are only nominally usable for communication and sensor processing in general. They fall totally short of acceptable performance for use in the presence of deliberate jamming efforts.
The injection of stray light, from whatever source into the laser cavity can cause intensity fluctuations, wavelength instability, spectrum bandwidth changes, increase in laser noise, threshold level variation, and optical waveform distortion. These effects can result in degraded system performance, and they can cause data rate reduction and an increase in the bit error rate (BER).
A number of investigators have looked at various aspects of these problems. Investigators have studied the laser characteristics due to reflected optical power, the nonlinear distortion performance of semiconductor laser diodes caused by coherent reflected light, and the effect of reflected optical power on BER performance for a 500 megabits per second system. In the presence of large feedback, laser noise causes a bit error rate saturation at a level several orders of magnitude greater than that without optical feedback. For the most part, these effects may be substantial for fiber optic communication and sensor transmissions. They are very substantial for free space transmissions such as satellite laser communications and in ground linking networks.
Attempts to minimize these problems have resulted in development of several optical isolators, especially for fiber optic systems. Bludau and Rossberg, "Low-Loss Laser-to-Fiber Coupling with Negligible Optical Feedback," Journal of Lightwave Technology, Vol. LT-3, No. 2, pp. 294-302, April 1985 suggested use of a miniature, spherically shaped, cylindrical lens as an interface between a laser and its transmission system. Kawano, et al., "A New Confocal Combination Lens Method for a Laser-Diode Module Using a Single-Mode Fiber," Journal of Lightwave Technology, Vol. LT-3, No. 4, pp. 739-745, August, 1985, utilized a miniature lens combination consisting of a spherical lens, a graded index (GRIN) lens, and a rod lens. This approach provides good laser-to-laser transmission coupling, but it does little with regard to optical isolation. Sugie and Saruwatari, "Distributed Feedback Laser Diode (DFB-LD) to Single-Mode Fiber Coupling Module with Optical Isolator for High Bit Rate Modulation," Journal of Lightwave Technology, Vol. LT-4, No. 2, pp. 236-245, February, 1986, developed an optical isolator consisting of a yttrium iron garnet (YIG) plate and an analyzer. Optical isolation is obtained by polarization. The YIG plate-analyzer approach offers an insertion loss of about 6 dB and a reflected light isolation of about 21 dB.
In each case, these devices are only marginally satisfactory for optical fiber coupling systems. They are totally inadequate to accommodate systems subjected to optical jamming, and they are not adequate to compensate for cross talk from a receiver channel where adjacent optical paths are utilized.
An optical isolator design for satellite laser communication systems or high speed analog and digital fiber links must exhibit high isolation from external optical injection. Also, the isolator must operate without distortion of the laser output in order to avoid a requirement for excessive correcting optics. An optical isolator exhibiting 50 dB or greater isolation and less then 5 dB insertion loss would eliminate the degrading effects of optical reflection and jamming signals.
This invention involves coupling the optical power from a diode laser to either a free space communication link's transmitting optics or to an optical fiber by means of a "notched" tapered lens. The tapered lens consists of core and cladding materials of differing indices of refraction.
A number of prior art devices have made use of tapered lenses but not for achieving optical isolation. The patent to Kapron et al., U.S. Pat. No. 3,779,628, discloses a light coupler for efficiently coupling a relatively large light source to an optical waveguide. The light coupler comprises a tapered core of transparent material and a layer of transparent cladding material disposed upon the surface of the tapered core, wherein the cladding material has a refractive index less than that of the tapered core material. The large diameter end of the tapered core is optically polished and is adapted to receive light from the source of optical wave energy. The small diameter end of the tapered core is substantially aligned with the core of the optical waveguide.
The patent to Khoe et al., U.S. Pat. No. 4,671,609, discloses a monomode optical transmission fiber with a tapered end portion for efficient coupling from a diode laser into the fiber core. A lens formed by immersing the tapered end portion of the fiber in a transparent liquid material is arranged on the end portion of the fiber. The lens has a higher refractive index than that of the fiber core. The lens is given a graded refractive index to reduce reflection loss and reduce feedback radiation to the diode laser.
The patent to Hirschfeld, U.S. Pat. No. 4,654,532, discloses an optical fiber gradually tapered from a relatively large diameter entrance pupil to a substantially smaller diameter at a position longitudinally displaced from the entrance pupil. The light beam traversing the fiber is angularly compressed by the taper of the medium in which it is confined. The gradual transition increases the input beam convergence gradually without exceeding the critical angle of the fiber; thus ideally, the taper of a fiber should not exceed 5.degree..
The patent to Sottini et al., U.S. Pat. No. 4,521,070, discloses a high efficiency laser light transmission device for high power radiation comprising a first optical fiber guide having a variable section coupled stiffly to the laser source, and a second optical fiber guide of uniform or variable section with the larger diameter near the output terminal. The two guides have a step index refraction distribution and at least a plastic material coating for strength and reliability. The second fiber guide is adaptable and replaceable depending on different use requirements; the first fiber guide is integrated with the laser source to form a "unique whole".
Although tapers have been used as power combiners to link a large diameter fiber to a small diameter fiber or a laser to a fiber, these devices are not designed to provide isolation between the laser and the rest of the system. The present invention can provide an isolation of up to 60 dB and an insertion/coupling loss of less than 3 dB.